In pulling of silicon single crystal by Czochralski method (the CZ method), a single crystal can be pulled up by pulling a seed crystal immersed in silicon melt. In particular, an external carbon heater is heated to a temperature of about 1450-1600 deg. C., silicon melt obtained by melting polycrystalline silicon raw material is held in a vitreous silica crucible, a seed crystal of a silicon single crystal is contacted with the surface of the silicon melt having a melting point temperature of about 1420 deg. C. and gradually pulled while being rotated, the seed crystal of silicon single crystal is grown as a core, and thus a silicon single crystal is manufactured gradually while controlling the pulling rate and the melt temperature. In order to maintain the solid-liquid interface of the silicon melt central part which is contacted with the single crystal close to the silicon melting point of 1420 deg. C., the temperature of the vitreous silica crucible is as high as 1450-1600 deg. C. In pulling of silicon single crystal which can last 2 weeks or more, the extent of deformation of a sidewall sagging of a rim portion of the vitreous silica crucible may be 5 cm or more.
In the pulling of silicon single crystal, firstly, the crystal in the center of this seed crystal is widened (shoulder formation) until the desired diameter is obtained. Next, a single crystal shaped as cylindrical ingot is pulled up by performing body pulling. Finally, a single crystal is pulled up by narrowing the bottom portion. In the vitreous silica crucible having a diameter of 610-1015 mm (silicon ingot diameter: 200 mm, 300 mm, 450 mm), a large size single crystal silicon ingot such as those having a length of 2 m or more is manufactured. The single-crystal wafer manufactured from such a large size ingot is suitable for manufacturing flash memory or DRAM.
Since price reduction and high performance of flash memory or DRAM are progressing rapidly, to respond to the demand, a large size single crystal silicon ingot is required to be manufactured with high quality and at low cost. Therefore, a large size crucible is required to be manufactured with high quality and at low cost.
Moreover, although the present process using a wafer having a diameter of 300 mm is mainstream, a process using a large diameter wafer having a diameter of 450 mm is in development. For this reason, in order to manufacture a large diameter wafer having a diameter of 450 mm stably, a large size crucible with high quality has been on demand increasingly. However, in the manufacture of a large size vitreous silica crucible, problems exist as follows.
A phenomenon that the surface of the silicon melt vibrates periodically can be seen during pulling of the silicon single crystal. This is called a melt surface vibration. When the melt surface vibration occurs, the seed crystal is difficult to be contacted with a flat silicon melt surface, and a silicon single crystal cannot be pulled up. In addition, when the melt surface vibration occurs during pulling, a dislocation occurs, and the silicon is polycrystallized, and therefore, it becomes a problem that it is unable to be all used as a product. In particular, in the processes of dipping of the seed crystal and the shoulder formation, which are initial processes in pulling of silicon single crystal, are likely to be affected adversely by melt surface vibration. The influence largely determines the quality of the pulled silicon single crystal ingot.
The cause of melt surface vibration is considered as follows. In general, the reaction of SiO2 (solid)→Si (liquid)+20 occurs on the interface between the silicon melt and the vitreous silica to dissolve the vitreous silica. It is considered that, due to, for example, an increase in the pulling temperature and decrease in the ambient pressure, a reaction of Si (liquid)+O→SiO (gas) occurs, and the melt surface vibrates as the SiO gas rises from inside of the melt. For a large size vitreous silica crucible, the distance from the outside carbon heater to the central part of the silicon melt is greater than the conventional ones (more than 500 mm, and conventionally it was about 300 mm), thus an increasing temperature of the carbon heater during pulling cannot be avoided. That is, with an increase in the diameter of the silicon ingot, the melt surface vibration problem increases when the carbon heater temperature during pulling is increased. For this reason, the silicon melt surface vibration associated with the temperature increase during pulling is intensified, which needs to be suppressed. Therefore, for the purpose of improving the single-crystal yield of the silicon single crystal, it is necessary to suppress melt surface vibration occurring in the silicon melt.
In order to solve the problem of melt surface vibration, for example, Patent Literature 1 discloses a crucible as follows: A vitreous silica surface made of a first component of silica sand is formed on the inner surface of a silica crucible having an opaque layer and a transparent layer, thereafter, a vitreous silica made of a second component of silica sand is scattered and melted, and a vitreous silica made of synthetic silica sand is formed on the inner surface of a corner portion as well as a bottom portion. A technique of adjusting the bubble content of the inner surface layer of the crucible in a region near the melt surface at the beginning of pulling to a certain range is disclosed. This is due to the fact that it has been found based on the same principle as that of boiling stone which suppresses explosive boiling, micro concaves and convexes suppress melt surface vibration of silicon melt.
Patent Literature 2 discloses a technique to suppress melt surface vibration of silicon melt filled in a vitreous silica crucible by providing micro concaves in the crucible inner face layer. This is due to the fact that it has been found based on the same principle as that of boiling stone which suppresses explosive boiling, micro concaves suppresses melt surface vibration of silicon melt.